Optical communication systems are becoming more and more widespread mainly due to their very large information carrying bandwidths. The growth and diversity of lightwave networks, such as Wavelength Division Multiplexed (WDM) networks are placing new demands on all aspects of optical networks including, for example, capacity management and provisioning, maintenance, and reliable and robust operation. In addition, the current trend in many carrier networks is to implement standard IP based networks to achieve convergence of traditionally separate voice and data networks. To this end, the use of Ethernet based equipment in implementing carrier networks is becoming increasingly common.
Currently, high capacity optical networks are constructed as rings and use WDM technology to achieve high bandwidth capacities. For example, WDM ring networks are commonly used in metropolitan area network (MAN) applications but can also be used in LANs and WANs.
A block diagram illustrating an example prior art optical ring network comprising a plurality of nodes is shown in FIG. 1. The optical network, generally referenced 10, comprises a plurality of nodes 12, labeled node #1 through #5, connected by optical fiber links 14 so as to form an optical ring network. The ring network is characterized by communications that take place from node to node. In this example network, only a single ring is shown such that communications proceeds in the clockwise direction only. A second ring can be implemented that carries communications between nodes in the opposite or counter-clockwise direction.
Wavelength division multiplexed (WDM) optical networks are particularly desirable because of their restoration capabilities and suitability for minimizing the optical fiber length for the interconnection of system nodes. A typical WDM optical ring network includes network elements with optical add/drop multiplexers (OADMs), whereby some optical channels are dropped, some are added and/or other channels are expressed or passed through.
In a ring topology each ring node is connected to exactly two other ring nodes. The OADMs are used to construct a ring network whereby adjacent OADMs are connected pair wise while the network nodes are situated to form a ring. In a ring network, any node can be reached from any other node using two physically separate paths, i.e. one traveling clockwise and the other counter clockwise. The opposite traveling paths are used to provide protection against route failures. The use of at least two parallel fibers with traffic flowing in opposite directions provides restoration capabilities in the event of a fiber cut.
An Add/Drop Optical Multiplexer (ADOM) functions to filter or drop one or more wavelengths transmitted on the ring. The optical technologies typically used for producing an ADOM can be placed in two main categories, namely: (1) those using fixed filtering, whereby an ADOM is produced for dropping and adding a fixed wavelength, and (2) those using tunable filtering, whereby an external control determines the wavelength of the dropped and added channel.
Normally, only a single wavelength of light is used to carry optical signals from one node to another. To increase the communications bandwidth of the network, however, it is common to transmit light signals having multiple wavelengths. Additional signal channels can be added, wherein each channel corresponds to a different wavelength of light, using well-known DWDM techniques.
As is common practice in DWDM optical networks, optical add/drop multiplexers (OADMs) are used to drop, add or express one or more optical channels. A block diagram illustrating a typical structure of an Optical Add/Drop Multiplexer (OADM) is shown in FIG. 2A. The OADM, generally referenced 20, comprises drop module 22 adapted to generate a drop channel 26 from the multi-wavelength input signal and an add module 24, incorporating an optical amplifier 23, adapted to add a channel 28 to the multi-wavelength output signal.
A problem associated with such types of optical networks is the losses incurred from the passive optical devices, such as filters, couplers, multiplexers, etc. The losses, which exist at every node on the network, can increase as the number of optical components increases, such as in networks with large numbers of nodes,
To overcome the problem of optical losses from passive components, active optical amplifiers are used along the optical ring to boost the weak optical signals. Commonly used optical amplifiers include Erbium Doped Fiber Amplifiers (EDFAs). The use of optical amplifiers, however, is problematic. The amplifiers function to boost not only the optical signals but also any noise present. In addition, the optical amplifiers add noise to the line in addition to the signal and to the noise already present.
In non-ring type networks, techniques are well known for reducing the effects of the noise. In an optical ring network, however, the use of active optical amplifiers causes noise accumulation from amplifier spontaneous emissions (ASE) and from other noise sources as well, and is commonly referred to as noise creep.
Signal graphs illustrating the phenomena of ASE and noise build up or amplifier noise accumulation are shown in FIGS. 2B through 2G. The graphs correspond to points A, B and C shown in FIG. 2A. Each node along the ring employs at least one OADM 20. FIGS. 2B, 2C and 2D correspond to the baseline optical signal levels along the ring at points A, B and C, respectively. Similarly, FIGS. 2E, 2F and 2G correspond to the optical signal levels after a complete revolution around the ring at points A, B and C, respectively.
In each figure, the relative amplitude is plotted as a function of frequency (i.e. wavelength). The five peaks 30 in relative amplitude correspond to five different wavelengths in use along the ring. For illustration purposes the OADM 20 corresponds to the lowest frequency. With reference to the figures, at point A, all five wavelengths are present in the signal. Since the drop module functions to filter out a single wavelength 32, the signal at point B has the first wavelength filtered out. The add module employs an active amplifier to boost the optical signal with the new channel added. Thus, the signal at point C comprises the five wavelengths amplified. In addition, however, the noise level is also amplified by an amount ΔN1 where N represents the noise added to the signal each loop around the ring.
FIG. 2E illustrates the optical signal at the same point A after traversing the loop. The wavelength peaks are present along with an elevated noise floor. The original signal as shown in FIG. 2B is indicated by dotted line 34. After wavelength filtering by the drop module, the first wavelength is removed as shown in FIG. 2F. A channel is then added and the resulting signal amplified as shown in FIG. 2G. As in FIG. 2D, the noise along with the signal is amplified. The noise level also rises by an additional amount ΔN2 to a level indicated by line 38. The noise level at point C from the previous loop is indicated by line 36 while the original noise level at point A is indicated by line 34.
Thus, after two loops around the ring, the noise level has increased to a level equal to ΔN1+ΔN2. Considering even small amounts of amplifier noise, it can be seen that the effects of noise creep can amount to significant levels of noise after only relatively few trips around the ring. Eventually, the noise caused by amplifier noise and other noise source accumulation increases sufficiently to saturate the amplifier and communications along the optical ring becomes impossible.
One prior art solution to this problem is to open the optical ring. A block diagram illustrating a prior art optical ring network that attempts to solve the amplifier noise accumulation problem is shown in FIG. 3. The example network, generally referenced 40, comprises five nodes 42, labeled node #1 through node #5, connected by link 48. The link between nodes #1 and #5 is severed leaving two stubs 46, 44. The problem of noise creep is eliminated since the optical signal begins and terminates within a single rotation.
A disadvantage of this solution is that the ring properties of the network are destroyed. Communications around the ring can only take place in one direction. Thus, one half of the bandwidth is lost. For example, in a closed ring, two nodes normally can communication with each other in two directions, clockwise and counter-clockwise directions of communications. The network 40, however, only supports unidirectional communications. Bi-directional communications is a very desirable characteristic and a major benefit of employing optical networks in ring configuration.
A solution to this is to normally maintain the ring in an open state and to close it only when necessary such as during a fiber cut or other failure along the ring. This requires adding means to the network operative to detect fiber cuts and to close the ring in response thereto.
An alternative prior art solution to the problem of noise creep is to break the loop and insert an electrical based repeater to regenerate the signal in the electrical domain. The repeater functions to convert the signal from optical to electrical and back to optical. A block diagram illustrating a prior art Optical Electrical Optical (OEO) termination module is shown in FIG. 4. The network, generally referenced 50, comprises a plurality of nodes 52, labeled node #1 through node #N, connected by optical links 54.
The ring is broken and an Optical/Electrical/Optical (OEO) termination is inserted. The OEO terminator comprises an optical demultiplexer 56, optical multiplexer 64, optical to electrical converters 58, electrical repeaters 60 and electrical to optical converters 62. In operation, the optical signal received by the demultiplexer is divided into N optical signals 57 each having a different wavelength. Each individual channel is then converted from the optical domain to the electrical domain by optical to electrical converter 58 to yield an electrical signal 66.
The electrical signal is then amplified and regenerated to yield a regenerated electrical signal 68. This signal is then converted to an optical signal 70 by electrical to optical converter 62. The optical signals output from the N converters 62 are multiplexed by multiplexer 64 into a composite multi-wavelength optical signal that is then transmitted to the first node on the ring.
A benefit of electrical regeneration of the signal is that the noise is cleaned from the ring. In addition, each individual channel is accessible electrically for any purpose. A disadvantage of this solution, however, is that it is relatively costly in terms of complexity and the requirement to add optical and electrical based hardware to the ring. The electrical based equipment must be managed, adding to the cost and complexity. In addition, the equipment typically consumes a large amount of space. Further, the use of additional electrical equipment lowers the overall reliability of the network as it is another potential point of failure.
Therefore, there is a need for a solution to amplifier noise accumulation in optical ring networks that does not require costly, complex electro/optical based hardware and that provides the bi-directional communications benefit of ring networks.